The present invention relates to a fixed pattern detection apparatus and fixed pattern detection method which detect a fixed pattern for establishing timing synchronization with a spreading code contained in a received signal in a CDMA (Code Division Multiple Access) communication type reception apparatus.
In the communication field, a fixed signal pattern is generally inserted into a signal on the transmitting side, and the correlation is calculated on the receiving side for the purpose of detection of a received signal and timing synchronization. In broadband CDMA excepted as a next-generation cell phone standard, signals obtained by spreading desired information by a user-specific spreading code are superposed and transmitted in the same frequency band. The signals are despread using a user-specific spreading code prepared on the receiving side, thereby extracting the original desired information from the received signals. Despreading must be synchronized with the spread spectrum timing, in other words, synchronization acquisition must be performed.
In synchronization acquisition, the transmitting side transmits a predetermined fixed pattern, and the receiving side calculates a correlation value between a received signal and the fixed pattern to detect synchronization. For example, in broadband CDMA, a mobile station accesses the base station of a handover destination cell by using a common control channel on the basis of information (transmission timing, spreading code, signature, or the like) notified by a BCH (Broadcast CHannel) received from the base station in soft handover.
A mobile communication system standard by the standardization project 3GPP (3rd Generation Partnership Project) for 3rd generation mobile communication systems defines PRACH (Physical Random Access CHannel) as a physical channel used at this time.
The PRACH has a 4096-chip long preamble field as a fixed pattern for spreading code synchronization detection. The mobile station arbitrarily selects one of 16 preset types of preamble signatures (16-chip length), and uses the selected signature repetitively 256 times. The mobile station executes complex spreading by a scrambling code notified by the base station, generating a preamble field.
The broadband CDMA system requires a very long fixed pattern because of a very low S/N ratio upon chip-rate conversion. The presence of the frequency drift degrades the reception characteristic to a non-negligible degree. In general, such a fixed pattern must be detected within a predetermined time in the fading environment, and a detection characteristic is also required. From this, demands have arisen for a fixed pattern detection apparatus which improves the detection characteristic on the premise that the hardware scale, cost, and power consumption are reduced and the restriction on the processing time is met.
The present inventor has proposed a fixed pattern detection apparatus disclosed in Japanese Patent Laid-Open No. 2001-136103 (reference 1) in order to shorten the fixed pattern detection processing time and reduce the circuit scale. FIG. 13 shows the fixed pattern detection apparatus proposed in reference 1. The conventional fixed pattern detection apparatus detects a fixed pattern C generated by spreading as shown in FIGS. 14A and 14B. The fixed pattern C is formed from a symbol sequence C0, C1, . . . , CN−1 having a code length of N chips (N is an integer of 2 or more). The fixed pattern C is generated by spreading each symbol of a signature pattern U by a spreading code S.
As shown in FIG. 14A, the signature pattern U used to generate the fixed pattern C is constituted by repeating a symbol sequence U0, U1, . . . , UK−1 having a length of K chips (K is an integer of 2 or more) M times (M is an integer of 2 or more) (K×M=N). The spreading code S is formed from an arbitrary N-chip long symbol sequence S0, S1, . . . , SN−1. In the fixed pattern C, each of the symbols U0, U1, . . . , and UK−1 Of the signature pattern U is spread to M symbols by the spreading code S.
In the 3GPP PRACH, the spreading code S corresponds to a scrambling code from the base station. The signature pattern U corresponds to one of 16 types of preamble signatures (K=16-chip length) registered in the mobile station in advance. As shown in FIG. 14B, the signature pattern U is repeated M=256 times and used as an I component (in-phase component) and Q component (quadrature component). The Q component is converted into an imaginary number j, and the imaginary number j is complex-spread by a corresponding spreading code S to generate a fixed pattern C.
The conventional fixed pattern detection apparatus detects a desired fixed pattern by performing despreading for a received signal in an order reverse to spreading. In FIG. 13, received signals 81A containing a fixed pattern are temporarily accumulated in a received signal memory 81, and sequentially read out as received signals C under the control of a received signal memory controller 82. The correlation between each received signal C and the spreading code S is calculated by a spreading code correlator 83.
At this time, spreading code sequences having a length N from a spreading code generator 84 are thinned out and rearranged every K chips. M spreading code sequences are supplied to the spreading code correlator 83 via a spreading code shift register 85. The spreading code correlator 83 calculates the correlations between the M spreading code sequences and M received signals C every K chips, calculates spreading code correlation values corresponding to K signature pattern symbols, and stores the correlation values in a correlation value memory 86. Received signals are read out from positions delayed by one chip from the received signal memory 81. The spreading code correlator 83 calculates spreading code correlation values A for a period of N+L chips including an indefinite time width of L chips.
Spreading code correlation values corresponding to K signature pattern symbols at the same delay position are read out from the correlation value memory 86 under the control of a correlation value memory controller 87. A signature pattern correlator 88 calculates the correlations between the spreading code correlation values and a signature pattern prepared in advance in a signature pattern table 89. A signature detector 90 selects a preferable correlation value from signature pattern correlation values obtained in correspondence with respective delay positions, and outputs the selected correlation value as a detection result 90A.
A spreading code correlation value is calculated by the spreading code correlator 83 shorter than the N-chip fixed pattern length. A final signature pattern correlation value is calculated by the signature pattern correlator 88. The processing time for calculating a desired correlation value is shortened.
When a plurality of signature patterns exist, the spreading code correlator 83 is shared between them, and a signature pattern correlator 88 constituted by parallel-arranging separate correlators corresponding to types of signature patterns is used. Whether a signature pattern has been detected can be simultaneously confirmed for a plurality of types of signature patterns. In this case, the circuit scale is reduced in comparison with a conventional arrangement using a plurality of correlators with a K×M-chip length in correspondence with signature patterns.
The conventional fixed pattern detection apparatus can be expected to shorten the processing time for calculating a correlation value and reduce the circuit scale. However, the conventional apparatus calculates correlation products at once for an N-chip fixed pattern length. If the frequency drift exists in a received signal, the reception characteristic degrades as the number of chips subjected to simultaneous correlation product calculation is larger.
If the frequency drift exists in a received signal, the phase of the received signal may be inverted at a low frequency corresponding to the frequency difference. When the number of chips subjected to simultaneous correlation product calculation is large and a received signal undergoes arithmetic processing for a relatively long period, a received signal influenced by phase inversion may be used at high probability. A good correlation cannot be obtained, resulting in a poor reception characteristic.